Differential pressure sensors including built-in pressure sensor chips, which include a sensor diaphragm that outputs a signal corresponding to the difference between pressures applied to one and the other surfaces of the sensor diaphragm, have been used as industrial differential pressure sensors.
Such a differential pressure sensor is structured so that measurement pressures applied to high-pressure-side and low-pressure-side pressure-receiving diaphragms are transmitted to one and the other surfaces of the sensor diaphragm by enclosed liquid that serves as a pressure transmitting medium. Strain of the sensor diaphragm is detected as, for example, a change in resistance of a strain resistance gauge, and the resistance change is converted into an electrical signal to be output.
The differential pressure sensor is used to, for example, measure a liquid surface height in a sealed tank containing measurement fluid in, for example, a high-temperature reaction column of a petroleum refining plant by detecting a pressure difference between two locations, which are upper and lower locations, in the sealed tank.
FIG. 12 is a schematic diagram illustrating the structure of a differential pressure sensor according to the related art. This differential pressure sensor 100 includes a pressure sensor chip 1, which includes a sensor diaphragm (now shown) and which is installed in a meter body 2. The sensor diaphragm included in the pressure sensor chip 1 is made of, for example, silicon or glass. A strain resistance gauge is formed on a surface of the diaphragm, which is thin plate shaped. The meter body 2 includes a main section 3, which is made of a metal, and a sensor section 4. Barrier diaphragms (pressure-receiving diaphragms) 5a and 5b, which serve as a pair of pressure-receiving portions, are provided on side surfaces of the main section 3. The pressure sensor chip 1 is installed in the sensor section 4.
In the meter body 2, the pressure sensor chip 1, which is installed in the sensor section 4, is connected to the barrier diaphragms 5a and 5b provided on the main section 3 through pressure damping chambers 7a and 7b that are partitioned from each other by a large-diameter center diaphragm 6. Pressure transmitting media 9a and 9b, such as silicone oil, are enclosed in channels 8a and 8b that connect the pressure sensor chip 1 to the barrier diaphragms 5a and 5b. 
The pressure media, such as silicone oil, are necessary to prevent adhesion of foreign matter contained in the measurement medium to the sensor diaphragm and to prevent corrosion of the sensor diaphragm by separating the pressure-receiving diaphragms, which are resistant to corrosion, from the sensor diaphragm, which is sensitive to stress (pressure).
As illustrated in FIG. 13A, which is a schematic diagram illustrating an operation mode of the differential pressure sensor 100 in a steady state, a first process fluid pressure (first measurement pressure) Pa is applied to the barrier diaphragm 5a, and a second process fluid pressure (second measurement pressure) Pb is applied to the barrier diaphragm 5b. Accordingly, the barrier diaphragms 5a and 5b are displaced and the applied pressures Pa and Pb are transmitted to one and the other surfaces of the sensor diaphragm included in the pressure sensor chip 1 through the pressure damping chambers 7a and 7b, which are partitioned from each other by the center diaphragm 6, by the pressure transmitting media 9a and 9b. As a result, the sensor diaphragm of the pressure sensor chip 1 is displaced by an amount corresponding to the difference ΔP between the pressures Pa and Pb.
If, for example, an excessive pressure Pover is applied to the barrier diaphragm 5a, the barrier diaphragm 5a is displaced by a large amount as illustrated in FIG. 13B, and accordingly the center diaphragm 6 is displaced so as to absorb the excessive pressure Pover. If the barrier diaphragm 5a comes into contact with the bottom surface of a recess 10a in the meter body 2 (excessive-pressure protection surface) so that further displacement of the barrier diaphragm 5a does not occur, transmission of the pressure difference ΔP to the sensor diaphragm through the barrier diaphragm 5a stops. If the excessive pressure Pover is applied to the barrier diaphragm 5b, similar to the case in which the excessive pressure Pover is applied to the barrier diaphragm 5a, the barrier diaphragm 5b comes into contact with the bottom surface of a recess 10b in the meter body 2 (excessive-pressure protection surface) so that further displacement of the barrier diaphragm 5b does not occur. Accordingly, transmission of the pressure difference ΔP to the sensor diaphragm through the barrier diaphragm 5b stops. As a result, breakage of the pressure sensor chip 1, that is, breakage of the sensor diaphragm included in the pressure sensor chip 1, due to the application of the excessive pressure Pover can be prevented.
Since the differential pressure sensor 100 is configured so that the pressure sensor chip 1 is disposed in the meter body 2, the pressure sensor chip 1 can be protected from an external corrosive environment including, for example, process fluid. However, the differential pressure sensor 100 is necessarily large in size because the recesses 10a and 10b, which limit the displacements of the center diaphragm 6 and the barrier diaphragms 5a and 5b, are provided to protect the pressure sensor chip 1 from the excessive pressure Pover.
Accordingly, a structure in which a pressure sensor chip is provided with a first stopper member and a second stopper member has been proposed. The first stopper member and the second stopper member include recesses arranged so as to face one and the other surfaces of a sensor diaphragm, so that the sensor diaphragm is prevented from being excessively displaced when an excessive pressure is applied thereto. Accordingly, the sensor diaphragm is prevented from being damaged or broken (see, for example, PTL 1).
FIG. 14 is a schematic diagram illustrating a pressure sensor chip having the structure described in PTL 1. FIG. 14 shows a sensor diaphragm 51-1, first and second stopper members 51-2 and 51-3 that are bonded together with the sensor diaphragm 51-1 interposed therebetween, and first and second bases 51-4 and 51-5 bonded to the stopper members 51-2 and 51-3. The stopper members 51-2 and 51-3 and the bases 51-4 and 51-5 are made of silicon or glass.
The stopper members 51-2 and 51-3 of the pressure sensor chip 51 respectively have recesses 51-2a and 51-3a formed therein. The recess 51-2a in the stopper member 51-2 faces one surface of the sensor diaphragm 51-1, and the recess 51-3a in the stopper member 51-3 faces the other surface of the sensor diaphragm 51-1. The recesses 51-2a and 51-3a have curved surfaces (non-spherical surfaces) that follow the sensor diaphragm 51-1 in the displaced state, and pressure introducing holes (pressure introduction holes) 51-2b and 51-3b are formed at the bottom portions of the recesses 51-2a and 51-3a, respectively. Pressure introducing holes (pressure introduction holes) 51-4a and 51-5a are formed in the bases 51-4 and 51-5, respectively, at positions corresponding to the positions of the pressure introduction holes 51-2b and 51-3b in the stopper members 51-2 and 51-3.
When the pressure sensor chip 51 is used, if an excessive pressure is applied to the one surface of the sensor diaphragm 51-1 and the sensor diaphragm 51-1 is displaced, the entirety of the displaced surface is received by the curved surface of the recess 51-3a in the stopper member 51-3. Similarly, if an excessive pressure is applied to the other surface of the sensor diaphragm 51-1 and the sensor diaphragm 51-1 is displaced, the entirety of the displaced surface is received by the curved surface of the recess 51-2a in the stopper member 51-2.
Thus, the sensor diaphragm 51-1 is prevented from being excessively displaced when an excessive pressure is applied to the sensor diaphragm 51-1, and stress concentration does not occur at a peripheral portion of the sensor diaphragm 51-1. Accordingly, the risk of breakage of the sensor diaphragm 51-1 due to the application of excessive pressure can be effectively reduced, and the excessive-pressure protection operation pressure (withstand pressure) can be increased. Furthermore, in the structure illustrated in FIG. 12, the size of the meter body 2 can be reduced by omitting the center diaphragm 6 and the pressure damping chambers 7a and 7b and directly transmitting the measurement pressures Pa and Pb from the barrier diaphragms 5a and 5b to the sensor diaphragm 51-1.